Steam generating and superheating unit with recirculated gas introduction at one endof hopper bottom furnace



Auz- 4,195.9

Filed Aug. 5,1954

RECIRCULATED GAS INTRODUCTION AT ONE END OF HOPPER BOTTOM FURNACE FIG-11.

INVENTOR Paul ff Jfoc/z BY ATTORNEY 3 Sheets-Sheet 1 4, 1959 P.- H. KOCH I swam GENERATING AND supamnmmc unnmm RECIRCULATED GAS mmonucwrou AT ONE END OF HOPPER BOTTOM FURNACE Filed Aug. 5, 1954- 3 Sheets-Sheet 2 as ea INVENTOR I I Y I BY jack F G. 3 w

- ATTORNEY 2,897,796 ERHEATING UNIT WITH Aug. 4, 1959' ,P.'H. KOCH, smm GENERATING AND suP RECIRCULATED GAS mmonucnon mom:

END OF HOPPER BOTTOM FURNACE 3 Sheets-Sheet 3 Filed Aug. '5, 1954 INVENTOR J zz/ a f f ach ATTORNEY are Paul H. Koch, Bernardsville, N.J., assignor to The Babcoek & Wilcox Company, New York, N.Y., a corporation of New Jersey Application August 5, 1954, Serial No. 448,061 4 Claims. (Cl. 122-478) Unite 5 This invention relates to steam generating and superheating units for power plants. 7

The invention is more particularly concerned with the maintenance of a predetermined final steam temperature over a wide range of operating conditions such as widely varying rates of steam generation.

The invention involves a high pressure and high capacity steam generating and superheating unit in which the predominant part, if not all, of the steam is generated in furnace wall tubes by the transmission of radiant heat thereto from burning fuel and combustion gases. The invention is concerned with control of the heat absorption by the furnace wall tubes by the introduction of recirculated gases. More specifically, the unit of the invention involves a steam superheater receiving the steam generated in the furnace wall tubes and heating that steam to a predetermined temperature, by the absorption of heat from the combustion products. The unit is of a type which, as the rate of vapor generation decreases to a low value, there would be material departures from a predetermined temperature of the superheated steam if no corrective influence was applied. This invention involves a particular method of utilizing the controlled flow of recycled furnace gases for preventing such temperature variations, or attaining by the coordination of the rate of flow of recycled furnace gases, a changing rate of vapor generation and changes in final steam temperature to correct for the latter changes and, in substance, maintain a predetermined total steam temperature over a wide load range.

Specifically, the invention involves a steam generating superheating unit of the radiant type having a vertically elongated furnace chamber, the walls of which include steam generating tubes, with the lower parts of these tubes arranged so as to define a hopper bottom for the furnace 'with the longitudinal axis of each hopper extending normal to the plane of the burned wall. This illustrative unit is fired by fuel burning means in a vertical wall forming a continuation of the end wall of the hopper and at an elevation substantially above the hopper, and the recirculated furnace gases are introduced into the furnace chamber through wall openings in or adjacent to the opposite end wall of the hopper and at an elevation lower than that of the fuel burning means. Since the recycled furnace gases, thus introduced, are at a temperature lower than the flame and other products of combustion from the fuel burning means, and, as the density of the introduced gases is greater than the density of the gases issuing from the fuel burning means, the conditions are such as to promote the filling of the adjacent lower part of the furnace chamber with lower temperature gases in such a way that the higher temperature gasses will be forced away from the heat absorbing elements in the lower part of the furnace chamber to thus decrease the rate of heat absorption to the steam generating tubes in that part of the furnace. This action, together with an enveloping effect of the recycled gases (on the flames or higher temperature gases) issuing from the fuel burning means and the atent 7 g mamas Patented Aug. 4:, 1959 interposition of recycled gases between those flames (or gasses) and the furnace wall elements absorbing radiant heat, decreases the heat absorbed by the vapor generating elements by decreasing the residence time of the higher temperature gases within the furnace and provides for an increase in the heat content of the heating gases, flowing from the fuel burning means through the furnace outlet, at a decreased or decreasing load. This increase in heat availability of the heating gases at the furnace exit increases the heat absorbed by the elements of the superheater beyond that exit and provides a compensating influence for the inherent tendency of the superheater to effect a final steam temperature less than that desired.

The invention in a more specific sense involves the above described components in combination with a provision of an air heater and a tubular connection between the cold air inlet of the air heater and the recycled gas system at a position near the furnace to prevent furnace gases from entering the recirculated gas outlet ports at high loads, when there may be no recycled gas flow.

The invention also involves a method of maintaining a predetermined temperature of superheated steam or vapor over a wide range of load (or vapor generating rate). This method is exemplified in the operation of the illustrative vapor generating and superheating unit which in volves separate fuel burning means disposed at a plurality of positions successively more remote from the gas exit of the furnace, in combination with a gas recirculation system for returning partially cooled heating gases from a position downstream of the superheater to a position in the furnace. This method involves selective burner operation wherein, as the load decreases, the burner or fuel firing means at a position most remote from the furnace gas exit is rendered inoperative while the fuel burning means at another position continues to operate, and while the flow of recirculated gases is increased. In a more specific sense, the invention additionally involves the projection into the furnace of the recirculated gases, in a direction opposite to the direction of the projection into the furnace of burning fuel by the fuel burning means which continues in operation. It also involves the introduction into the furnace of the recirculated gas at a position in the furnace more remote from the furnace gas exit than the position at which fuel is being burned. When the position of recirculated gas introduction into the furnace is also at an elevation considerably less than the elevation at which fuel is being burned, the illustrative introduction of recirculated gas not only tends to fill the bottom of the furnace with gases of a temperature much lower than the temperature of the gases in the fuel burning zone, but the action of the fuel burning, combined with the recirculated gas introduction, tends to create an envelope of the lower temperature recirculated gases, around the flame or combustion elements projected into the furnace at the fuel burning zone. Both of these actions have the effect of decreasing the radiant transmission of heat to the vapor generating wall tubes of the furnace by the interposition of the lower temperature gases, relative to the heat absorbing surfaces and the gaseous heat emitting medium. The enveloping action also decreases the furnace heat absorption by reason of the decrease in surface area of the heat transmitting medium formed by the higher temperature gases.

In the illustrative method, the introduction of the recirculated gases at the described position, relative to the fuel burning, promotes the effectiveness of the lower temperature gases in decreasing furnace heat absorption by minimizing gas mixing; such mixing is also minimized by the low velocity of the recirculated gas introduction and by the distribution of such gas introduction relative to the pertinent heat absorbing surfaces, and relative to the distribution of fuel burning.

The above described action is effective, in combination with the consequent increase in superheater heat absorption, to change the ratio of heat absorbed in vapor generation to the heat absorbed in superheating the generated vapor, to the end that a predetermined superheat vapor generated temperature is maintained under changing load.

The invention also involves a particular recycled gas furnace inlet construction to prevent overheating, and to desirably coordinate recycled gas distribution and velocity at the inlets.

The invention is specifically set forth in the claims but, for a more complete understanding of the invention, and the advantages of its use, recourse should be had to the following description which refers by similar reference characters to the accompanying drawings.

In the drawings:

Fig. 1 is a sectional side elevation of the illustrative high temperature and high pressure steam generating and superheating unit.

Fig. 2 is a partial vertical elevation on the line 2-2 of Fig. ,1;

Fig'. 3 is a partial horizontal section on the line 3-3 of Figure 1, showing the manner in which some of the rear wall tubes at the rear end of the hopper have portions bent out of their wall alignment in order to provide for the passages through which the recycled furnace gases pass into the furnace chamber.

Fig. 4 is a horizontal section on the line 4-4 of Fig. 2 showing a ductwork construction near the ports through which the recirculated gas passes into the furnace.

Fig. 5 is a view similar to Fig. 2 illustrating a modified furnace construction; and

Fig. 6 is a horizontal section on the line 66 of Fig. 5.

The furnace chamber 10 has a front Wall 12 including steam generating tubes 14 leading from lower header 16, past three horizontal rows of burners 1820, to the steam and water drum 22. At the opposite wall of the furnace chamber, steam generating tubes 24 lead upwardly from the header 26, along the wall 28, to the arch 30, wherein these tubes are bowed partly across the furnace chamber, as illustrated. From the nose of the arch 30, some of these tubes extend upwardly as screen tubes 32 across the furnace gas exit to the furnace chamber roof 34. They then extend along this roof and through connections 36, to the drum 22. Others of the steam generating tubes extending upwardly from the header 26 have portions 38 disposed along the bottom of the superheater gas pass 40. At the exit 42 of this gas pass, the steam generating tubes are bent upwardly as indicated at 44 to form a screen through which the furnace gases pass to the gas turning space 46 at the top of the down gaspass 48. From the exit of the superheater gas pass the steam generating tubes extend along the roof portions 50 and 34 to connections with the drum 22. Others of the steam generating tubes along the wall 28 have their upper ends connected to the intermediate header 52 from which other wall tubes 54 lead past the exit 42 of the superheater gas pass and to an upper header 54. From this header steam and Water mixtures are conducted to the drum 22 through circulators 56 and 58.

The illustrative unit is shown in Figure 2 as having its furnace chamber divided into two parts by the division wall 60 formed by steam generating tubes leading upwardly from the headers 62 and 64 and connected to the drum 22 through appropriate headers and tubes. In the upper part of the furnace chamber some of the division wall tubes 60 have parts 66 bent out of their wall formation to the right, as shown, and other parts of other tubes have parts bent to the left, as shown at 68, for the purpose of pressure equalization by establishing gas communication between the two parts of the of vapor generation.

furnace From the hopper bottom header 64 other steam generating tubes 70 lead along the hopper wall 72 and thence upwardly along the wall 74 to an upper side wall header 76, Fig. 1. This header is in communication with the drum 22 by circulators 78. Similarly, other steam generating tubes along the furnace wall 82 have lower portions 84 inclined along the hopper bottom wall 86, as clearly indicated in Fig. 2. These tube portions are connected at their lower ends to a header 62 similar in structure and arrangement to header 64. Also, the tubes 80 are connected to the drum 22 in a manner similar to the connections for tubes 74.

Steam flows from the steam space of the drum 22 through the steam offtakes and 92 along the setting roof 94 to a header 96 from which part of the steam flows through the superheater supply tubes 98 leading to the right from that header and then downwardly along the wall portion 100 and the succeeding wall portions 102 and 104 to the superheater inlet header 106. Other superheater supply tubes 1 10 lead immediately downwardly from the header 96 and then across the roof 112 above the gas turning space 46 and then downwardly along the wall sections 102 and 104, to the superheater inlet header 106. From this superheater inlet header 106- superheater tubes 112 lead upwardly along the wall section 104 and then through the return bend banks 114-116 of tubes of the primary superheater. From the last of these banks of tubes the steam flows through the outlet tubes 118 of the primary superheater to the header 120' from which the steam flows through an attemperator 122 to the inlet header 124 of the secondary superheater 126. This superheater is shown as consisting of upright multilooped nested return bend tubes having their outlets connected to the header from which the steam, at its final temperature, may pass to a point of use.

Below the primary superheater in the gas pass 48 is an economizer including the banks of tubes 132-134. Feedwater is passed by appropriate means to the inlet header 136 of the economizer and the heated feedwater flows from the economizer outlet header 138, through appropi'iate connections to the drum 22.

The heating gases, after passing over the banks of tubes 132-134 of the economizer, pass through the duct 140 to an air heater (not shown). From the duct 140 a lateral duct 142, dampered at 143, conducts some of the partially cooled gases to the inlet of a fan 144, the outlet of which is connected to the duct components 146 and 148, communicating with the openings 164 and 166 which are indicated in Figs. 2 and 3, as provided for the introduction of recycled gas into the hopper bottoms of the two furnace chambers. The rate of flow of recirculated gas through the components 142, 144, 146

and 148 is controlled by suitable control devices for fan speed and/ or direct flow restriction in response to selected variables such as, for example, the steam temperature at or beyond the superheater outlet header 130 and the rate Such control mechanism is intended to be so operable that, as the rate of vapor generation drops due to decreased requirements, the rate of the flow of recycled gases into the hopper bottoms of the furnace will be increased to an extent sufficient to maintain the predetermined final temperature of superheated steam. Dampers such as may be directly affected or regulated by the pertinent variables.

Fig. 3 of the drawings shows some of the vapor generating wall tubes 24 bent inwardly of the furnace in the Zone of recycled gas entry to form openings between the tubes for the flow of that gas. As shown, the tube portions 24a, 24b, 24c and 24d are bent outwardly of the furnace so as to leave spaces between adjoining tubes, such as 24e and 24 for gas flow. The gas flow, for instance, between the tube portions Me and 24), is divided by the bent out portion 24a so as to provide two openings 149 for gas flow. In order that the recycled gas flow may have the desired velocity and in order that it may be properly distributed across the furnace, some of the openings between the tubes, such as 24 and 24g, are closed by the welding of flat studs 151 and 153 to the tubes 24 and 24g. As shown, the alternate spaces are, at least, partially closed off by such studs or stud plates, and the extent of this stud plate construction would be controlled by the desired distribution and velocity of the recycled gases.

Another factor involved in the provision of the par- .ticular recycled gas inlet construction shown in Fig. 3 is the protection of the inlets against overheating when the recycled gas flow is at a minimum.

In order to prevent the reverse flow of furnace gases through the recycled gas system when the latter is inoperative, the recycled gas duct component 148 is connected by a conduit 155 with the cold air inlet of the air heater, the control of air flow from this duct being effected by the damper 157.

Fig. 4 shows the ductwork component 148 near the furnace wall 28, to have two divergent branches 160 and 162 leading to the port areas 164 and 166 where the recirculated gases flow into the furnace as indicated by the arrows 168 and 170. This divided duct construction promotes uniform distribution of the flow of recirculated gas to the two sections of the furnace, and such distribution is also enhanced by the direction plates, or division plates 172 and 174 disposed within the duct branches 162 and 160, respectively, in the manner illustrated. This construction also functions as a deterrent to the reverse movement of recirculated gases from one section of the furnace into the duct and thence to the other section of the furnace when the recirculated gas velocity is low.

In the modification shown in Fig. 5 there are two furnace chambers 180 and 182 separated by a division wall 184, similar to the wall 60 of the Fig. 2 embodiment. In the rear wall 28 just above the hopper bottoms 186 and 188 there are port areas 190 and 192 extending entirely across the width of the corresponding hopper and with the port wall tubes arranged as indicated at 194, 195 and 196 to permit the flow of recirculated gases to be distributed over the entire widths of the pertinent furnace walls. Such distribution of the recirculated gas flow is indicated by arrows 198 in Fig. 6. The ductwork of the Fig. 5 and Fig. 6 embodiment indicated generally at 200, includes a main duct component 202 divided by a partition plate construction 204 aligned with the furnace division wall 184 and splitting the gas flow from the duct component 202. Further division of the recirculated gas fiow is effected by the auxiliary deflector plates, or division plates, 206 and 208 so that immediately externally of the furnace wall tubes 194 there are four recirculated gas furnace inlet chambers 210213. Each of these chambers is provided with a conduit preferably connected with the cold air inlet of the air heater, in a manner similar to the connection of the conduit 155 to the duct component 148, in Fig. l. The outlets of these conduits are indicated at 220-223.

Certain features of my invention are disclosed in my copending applications, S.N. 167,073, filed June 9, 1950, now US. Patent 2,737,931, and SN. 264,029, filed December 29, 1951, now US. Patent 2,819,702.

While in accordance with the provisions of the statutes I have illustrated and described herein the best forms of the invention now known to me, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by my claims, and that certain features of my invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. A vapor generating and superheating unit comprising vertical front, rear and side walls defining a vertically elongated furnace chamber, wall means forming a hopper at the bottom of said furnace chamber having its ends defined by said front and rear walls and its longitudinal axis normal to the plane of said front and rear walls, vapor generating tubes lining the side and end walls of said furnace chamber and hopper, fuel burning means in said front wall at an elevation above the upper end of said hopper and arranged to discharge burning fuel in a direction substantially parallel to the longitudinal axis of said hopper, a gas pass opening to the upper part of said furnace chamber, a convection superheater in said gas pass, and a gas recirculation system including gas discharge ports in said rear wall at an elevation below the level of said fuel burning means and adjacent the upper end of said hopper, and means for withdrawing heating gases from said gas pass downstream of said superheater and discharging the withdrawn gases through said gas discharge ports.

2. A vapor generating and superheating unit comprising vertical front, rear and side walls defining a vertically elongated furnace chamber, wall means forming a hopper at the bottom of said furnace chamber having its ends defined by said front and rear walls and its longitudinal axis normal to the plane of said front and rear walls, vapor generating tubes lining the side walls of said furnace chamber and hopper, fuel burning means in said front wall at an elevation above the upper end of said hopper and arranged to discharge burning fuel in a direction substantially parallel to the longitudinal axis of said hopper, a gas pass opening to the upper part of said furnace chamber, a convection superheater in said gas pass, and a gas recirculation system including gas discharge ports in said rear wall at an elevation below the level of said fuel burning means and above the upper end of said hopper, and means for withdrawing heating gases from said gas pass downstream of said superheater and discharging the withdrawn gases through said gas discharge ports.

3. A vapor generating and superheating unit comprising vertical front, rear and side walls defining a vertically elongated furnace chamber, wall means forming a hopper at the bottom of said furnace chamber having its ends defined by said front and rear walls and its longitudinal axis normal to the plane of said front and rear walls, vapor generating tubes lining the side walls of said furnace chamber and hopper, fuel burning means in said front wall at an elevation above the upper end of said hopper and arranged to discharge burning fuel in a direction substantially parallel to the longitudinal axis of said hopper, a gas pass opening to the upper part of said furnace chamber, a convection superheater in said gas pass, and a gas recirculation system including gas discharge ports in said rear wall at an elevation below the level of said fuel burning means and adjacent the upper end of said hopper, means for withdrawing heating gases from said gas pass downstream of said superheater and discharging the withdrawn gases through said gas discharge ports, and means for introducing cooling air into said gas recirculation system adjacent said gas discharge ports.

4. The combination of claim 1 further characterized by the fact that the inlet of the gas recirculation system receives furnace gases from a position beyond the superheater and directs the recirculated gases into the hopper bottom at an elevation below the elevation at which the lower portion of the hopper bottom vapor generating tubes begin to converge downwardly.

Jackson Oct. 18, 1938 De Baufre Jan. 28, 1941 (Other references on following page) Great Britain July 24, 1940 8 371,985 Italy June 12, 193,9 503,778 Belgium June 30, 195.1 516,599 Belgium Jan, 15, 1953 OTHER REFERENCES Journal of the Iren and Steel Institute of August 1947, pages 547 through 551. 

